The actors behind Marvel's biggest superheroes have teamed up in a heartwarming bid to brighten the day of a young fan with cancer.

Led by Deadpool star Ryan Reynolds, a host of Marvel actors including Chris Evans, Chris Pratt, Hugh Jackman and Chris Hemsworth have answered a social media plea to get in touch with 17-year-old Aussie Josh Rigby.

His mum Donna Rigby asked them to send words of encouragement to her terminally ill son "while we still have him with us."

Josh has been battling leukemia since he was five and was also diagnosed with glioblastoma at 15. It is the same form of brain cancer that Senator John McCain died from, according to the family's GoFundMe page.

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Reynolds, who referred to himself as "Uncle Deadpool", was the first to reply, telling the teen: "You have more courage and more strength in your pinky finger than any superhero does in their entire body."

He then got in touch with his fellow Marvel heroes, who soon followed suit.

Chris Evans jokingly gave Josh a hard time for liking Deadpool more than Captain America, while Chris Pratt added: "I think real heroes are young people like you have to face challenges and do so with the bravery... that you do."

The celeb that really made Josh's day was fellow Aussie Chris Hemsworth - Josh's family posted the Thor star's video to social media saying: "When josh saw you come up on screen at his fundraiser he yelled out "F*** YEAH!"...thanks for putting a smile on joshys face."

The videos were reportedly sent directly to the family, who later shared them on social media.